High viscosity aqueous crosslinked gels are used in a variety of operations and treatments carried out in oil and gas wells. Such operations and treatments include, but are not limited to, well completion operations, fluid loss control treatments, production stimulation treatments, formation permeability conformance operations and treatments to reduce water production.
An example of a production stimulation treatment utilizing a high viscosity crosslinked gelled fluid is hydraulic fracturing. In hydraulic fracturing treatments, the high viscosity fluid is utilized as a fracturing fluid and also carries particulate propping agents, e.g., sand, into the fractures formed. That is, the fracturing fluid is pumped through the wellbore into a formation to be stimulated at a rate and pressure such that fractures are formed and extended in the formation. The propping agent is suspended in the fracturing fluid so that it is deposited in the fractures when the gel is broken and returned to the surface. The propping agent functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the wellbore.
Borate ion has long been used as a crosslinking agent for forming high viscosity crosslinked gelled aqueous well treating fluids. Various sources of borate have been utilized including boric acid, borax, sodium tetraborate, slightly water soluble borates such as ulexite, and other proprietary compositions comprised of boric acid and dimers and trimers of borate ions. Different borate ion sources have different properties which affect their use as crosslinking agents. For example, the rate of crosslinking can vary depending on the borate ion source used.
Instant crosslinkers provide for quick increase of viscosity or gelling of the treatment fluid. Delayed crosslinkers provide a slower gelling of the treatment fluid. Depending on the delayed crosslinker, it can take on the order of a minute, several minutes or even an hour or more for the viscosity to increase to suitable levels (“gelling time”). Ulexite has been used as a delayed crosslinker. Depending upon its carrier fluid, it can have a gelling time on the order of about 15 minutes to about an hour. For some well operations requiring a delayed crosslinker, this gelling time is too long.